The manual doesn't really make it clear that the PECL ext is only for php4. For php5, you have to use the --with-tidy option. (At least, this was the case for me with php 5.2.5 on Mac OS 10.4 - Tiger.) Tiger ships with tidylibs already installed in /usr/include but there is broken header file. The easiest way to get php to compile with Tidy on 10.4 (and 10.5 Leopard also) is to download Macports and use that to install tidy (unless you want to build tidy from src.) After installing Macports via the .dmg, just su root and do:
port install tidy
(port is installed in /opt/local/bin)
Then configure --with-tidy=/opt/local (Macports installs stuff in /opt/local).
You can also use Macports to install all sorts of other libs such as png, libmcrypt, freetype and jpeg (although libpng and libjpeg are also available as package installs.)
The specific compile error is:
In file included from /usr/include/tidy/tidy.h:70,
/usr/include/tidy/platform.h:515: error: duplicate 'unsigned'
/usr/include/tidy/platform.h:515: warning: useless type name in empty declaration